blocks|key|695734|text|什么是用户定义的类加载器？|type|blockquote|depth|inlineStyleRanges|entityRanges|data|695735|用户定义的类加载器是扩展java.lang.Classloader或子类的类(标准Java库或第三方库不提供)。|unstyled|offset|length|style|CODE|695736|程序员是否编写用户定义的类加载器？|695737|是。|695738|它在jvm中的工作方式|695739|就像其他的类加载器一样。更具体地说，要加载一个类，它将类文件加载到一个byte[]中，然后调用由ClassLoader类实现的defineClass方法。其他事情可能需要委托给父类加载器。|695740|为什么要这样做？|695741|如果您需要类加载程序与普通类加载器不同的行为，则需要这样做。例如，您可能希望转换字节码，或者从标准实现无法处理的地方加载字节码。或者您可能希望实现与正常模式不同的资源加载。|695742|请试着用一种简单易懂的方式来解释，这样我才能准确地理解它是什么。|695743|我建议您阅读这篇关于类加载器的IBM教程：http://www.ibm.com/developerworks/java/tutorials/j-classloader/j-classloader.html+--它设计得很容易理解，并包含了一个定制类加载器的工作示例。(它有点老了，但这不重要。)|695744|或者，有很多选择.谷歌会告诉你的。|695745|如果您只想了解类加载器，请阅读：什么是Java+ClassLoader？|695746|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ibm.com/developerworks/java/tutorials/j-classloader/j-classloader.html|1|https://stackoverflow.com/questions/2424604/what-is-a-java-classloader^0|0|C|L|0|0|0|0|Z|6|1C|B|1R|B|0|0|0|0|L|29|0|0|0|G|K|1|0^^$0|@$1|2|3|4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1D|8|@$E|1E|F|1F|G|H]]|9|@]|A|$]]|$1|I|3|J|5|6|7|1G|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|1H|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1I|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|1J|8|@$E|1K|F|1L|G|H]|$E|1M|F|1N|G|H]|$E|1O|F|1P|G|H]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1R|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1S|8|@]|9|@]|A|$]]|$1|W|3|X|5|D|7|1T|8|@]|9|@$E|1U|F|1V|1|1W]]|A|$]]|$1|Y|3|Z|5|D|7|1X|8|@]|9|@]|A|$]]|$1|10|3|11|5|D|7|1Y|8|@]|9|@$E|1Z|F|20|1|21]]|A|$]]|$1|12|3|-4|5|D|7|22|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19]]|1A|$5|15|16|17|A|$18|1B]]]]

<blockquote>
 What is user-defined classloader?
</blockquote>

A user-defined classloader is a class (that is not provided by the standard Java libraries or 3rd-party libraries) that extends <code>java.lang.Classloader</code> or a subclass.

<blockquote>
 Does programmer writes user-defined classloader?
</blockquote>

Yes.

<blockquote>
 how it works in jvm 
</blockquote>

Just like any other classloader. More specifically, to load a class, it loads the classfile into a <code>byte[]</code> and then calls the <code>defineClass</code> method implemented by the <code>ClassLoader</code> class. Other things may entail delegating to a parent classloader.

<blockquote>
 and why it is required?
</blockquote>

It would be required if you need your classloader to behave differently to a normal classloader. For example, you might want to transform the bytecodes, or load them from a place that the standard implementations cannot cope with. Or you may want to implement resource loading differently to the normal pattern.

<blockquote>
 Please try to explain in a easy to understand way so I can exactly understand that what it is. 
</blockquote>

I suggest you read this IBM tutorial on classloaders: <a href="http://www.ibm.com/developerworks/java/tutorials/j-classloader/j-classloader.html" rel="nofollow noreferrer">http://www.ibm.com/developerworks/java/tutorials/j-classloader/j-classloader.html</a> It is designed to be easy to understand, and includes a worked example of a custom classloader. (It is a bit old, but that shouldn't matter.)

Alternatively, there are lots of alternatives ... as Google will tell you.

And if you just want to understand classloaders in general, read: <a href="https://stackoverflow.com/questions/2424604/what-is-a-java-classloader">What is a Java ClassLoader?</a>

blocks|key|182998|text|我会尽量保持简单。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|182999|Java使用类加载器在JVM中加载类。Java能够从文件系统上的二进制文件(*.class文件)或jar文件中的*.class文件中加载类。|183000|ClassLoader具有"loadClass“功能--它获取类的字符串名称(包含包和所有内容)并返回class+--一个描述该对象的类文件。一旦加载，该类就可以在JVM中使用。|183001|示例(伪代码)：|183002|ClassLoader+myCl+=+...
Class<Foo>+fooClass+=+myCl.loadClass("some.package.Foo")|code-block|syntax|javascript|183003|你为什么还要为类加载器操心呢？好吧，通常你不会真正创建自己的类加载器。有时候你需要它们。|183004|让我们说，您正在创建应用服务器/web服务器。可以在运行时加载应用程序，应用程序不应干预。在本例中，您可能希望实现自己的类装入器，并在自己的类加载器的帮助下加载每个WAR+(这在本质上是JBoss、tomcat等中的实际情况)。|183005|另一个例子，假设您将类定义存储在一些“奇怪”的地方，比如数据库，您可以通过网络获得类定义。在本例中，您将实现类加载器，它获取类定义，然后加载。|183006|最后一个音符。您自己的类加载器不应该真正“加载”类本身，通常它只负责带来字节流，然后调用其父方法来执行实际加载(所有类加载器都扩展了java.lang.ClassLoader)。|183007|当然，这是一个非常广泛的主题，在这篇文章中，我只是试图给出一个大致的方向开始。您可能应该阅读一些关于类加载器的文档。|183008|例如：这个应该是好的|offset|length|183009|霍普，这有帮助|183010|entityMap|0|LINK|mutability|MUTABLE|url|http://www.javaworld.com/article/2077260/learn-java/the-basics-of-java-class-loaders.html^0|0|0|0|0|0|0|0|0|0|0|3|7|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1B|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1C|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|1D|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|1E|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|1F|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1H|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1I|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1J|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1K|8|@]|9|@$Y|1L|Z|1M|1|1N]]|A|$]]|$1|10|3|11|5|6|7|1O|8|@]|9|@]|A|$]]|$1|12|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19]]]]

I'll try to keep it simple. 

Java uses classloaders to load classes inside JVM. Java is able to load classes from binaries (*.class files) on filesystem or *.class files inside jar files.

ClassLoader is something that has a "loadClass" functionality - it gets a string name of the class (with package and everything) and returns Class - a class file that describes this object. Once loaded, the class can be used inside the JVM.

Example (pseudocode):

<pre><code>ClassLoader myCl = ...
Class&lt;Foo&gt; fooClass = myCl.loadClass("some.package.Foo")
</code></pre>

Why would you even bother about classloaders? Well, usually you don't really create your own classloaders. Hoewever sometimes you need them. 

Lets say, you're creating application server/web server. It's possible to load the application in runtime, applications should not interfere. In this case you might want to implement your own class loader and load each WAR with the help of its own classloader (this is essentially what actually happens in JBoss, tomcat and so forth).

Another example,
Lets say you store your class definitions in some "weird" places - like database, you you get the class definitions via network. In this case you'll implement your classloader that fetches the class definition and then loads.

And the last note. Your own class loader shouldn't really "load" the class by itself, usually its only responsible to bring the byte stream and then call its parent's method to do actual loading (all class loaders extends java.lang.ClassLoader).

Of course its a very broad theme to be covered in this post, I just tried to give a general directions to start with. You should probably read some documentation about classloaders.

For example: <a href="http://www.javaworld.com/article/2077260/learn-java/the-basics-of-java-class-loaders.html" rel="nofollow">this one should be good</a>

Hope, this helps

I am reading about Jvm architecture but i could't understand class loader architecture in it.I understood that what a bootstrap classloader is , but could't understand working of user-defined classloader , how it works in jvm and why it is required?

Does programmer writes user-defined classloader? I have been working on java but have not used or could't see when it's used? Please try to explain in a easy to understand way so I can exactly understand that what it is.

What is user-defined classloader?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在阅读有关Jvm体系结构的文章，但我无法理解其中的类加载器体系结构，我理解引导类加载器是什么，但不能理解用户定义的类加载器的工作原理，它在jvm中如何工作，以及为什么需要它？程序员是否编写用户定义的类加载器？我一直在研究java，但是没有使用它，或者看不出它何时使用？请试着用一种简单易懂的方式来解释，这样我才能准确...

问什么是用户定义的类加载器？
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